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"2010-04-16 12:03 PM EDT -0.03 knots Slack, Ebb Begins"
Meaning that, in theory, the ebb current (southbound in NYC's Hellgate) will begin just after noon. You have to do some juggling and make some decisions about how you want to transit the HG and the East River itself and note there's limited clearance under the bridge in the east channel passing Roosevelt Island, and foul bottom in the holding areas marked as anchorages nearby.
Slack tends to be an hour long OR LESS, so if ebb begins at noon, and you are heading south, you might want to leave yourself some slack (no pun intended) by hitting the HG around 11AM and passing through at slack water, or with minimal current against you.
Once the ebb begins, you can use the current to get a speed boost running down the East River BUT the problem there is that current can hit six knots, and you've got no brakes, no way to slow down, if you have any problems en route. In the ER itself, you can find oddities like wind shifts through Manhattan that make sailing quite interesting (yes, sailing is legal there) and square standing waves 2-3 tall as the water bounces off the bottom in odd ways. So making a slower transit may not be a bad thing, if you have any doubts in the engine or are shorthanded.
You may also find there's quite a crowd coming through at slack, ranging from gasoline barges that have no vision OR brakes, to powerboats who really don't care about you being waked.
This is not to scare anyone, just to say you need to have your eyes open. One day a friend got stuck at the office and we couldn't make slack...and we spent the next three hours making about two hundred yards or progress against the current above the HG. But if you time it right and you know what to expect, it's a piece of cake, no scary monsters at all. Time it wrong and have the engine crap out...not so good.